This invention relates to semiconductor optical devices such as optical switchboards in an optical communication systems, optical modulators used as light sources for transmission and local oscillation, optical switches, semiconductor lasers, etc., and more particularly to a semiconductor optical device comprising an improved optical waveguide.
Of various semiconductor optical devices, the semiconductor laser has been described in many publications, for instance, Electronics Letters, Vol. 22, No. 5, pp. 249-250 (1986) by A. R. Adams. The publication includes a qualitative discussion on the use of a strained-layer superlattice for an active layer of a semiconductor laser to achieve a decrease in intervalence band absorption. It is possible, by use of a strained-layer superlattice for the active layer of a semiconductor laser, to reduce the threshold current of the laser and to increase the modulation rate. In the description of the publication, however, the strained-layer superlattice is used only for the active layer.
Carrier injection type optical switches, also, have been described in a number of publications, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 60-134219 (1985). The carrier injection type optical switch described in the publication comprises an optical waveguide using a bulk crystal. U.S. Pat. No. 4,737,003 corresponds to the Japanese Patent Application Laid-Open (KOKAI) No. 60-134219 (1985).
Furthermore, Japanese Patent Application No. 61-215806 (1986) discloses an optical waveguide using a superlattice. In this case, however, the superlattice has lattice matching and is not a strained-layer superlattice. While InGaAsP/InP, for example, is generally used for the superlattice in the prior art mentined above, the InGaAsP layer is in lattice matching with the InP layer, and the superlattice is not a strained-layer superlattice.
InGaAsP/InP is used as a semiconductor laser material conforming to a wavelength used for optical communication, for instance, 1.3 .mu.m or 1.55 .mu.m. InGaAsP is used for the active layer, and InP for the substrate or the cladding layer of the semiconductor laser. In addition, avalanche photodiodes (APD) are also produced using InGaAsP/InP. Thus, an InGaAsP/InP system is used in most of semiconductor optical devices which are now in practical use for optical communication. In all these semiconductor optical devices, however, the InGaAsP layer and the InP layer are in lattice matching with each other, and are not provided as a strained-layer system.
FIG. 2 is a diagram illustrative of the band structure of the above-mentioned InGaAsP/InP used as a conventional optical waveguide.
In this material system, the curvature of the heavy-hole band is small, as shown in FIG. 2. The holes are therefore distributed also in the region of greater wave numbers, as shown in FIG. 2, so that electrons in the split-off band will be easily excited by optical absorption. Accordingly, the optical absorption between the split-off band and the heavy-hole band, namely, the intervalence band absorption is large, and the absorption is conspicuous especially in longer wavelength regions. Besides, a large loss due to the absorption in a higher hole concentration layer has been the cause of a reduction in the performance of carrier injection type optical modulators, semiconductor lasers with an external cavity, wavelength-tunable semiconductor lasers, etc.
Moreover, the small curvature of the heavy-hole band, as shown in FIG. 2, means a high density of states. Upon carrier injection, therefore, the change in the carrier energy distribution (called "the band-filling effect") is slight, and the change in refractive index is small. Accordingly, it is necessary to reduce the angle of the optical waveguide intersection in an optical crosspoint switch, with the result of a greater device width or a closer arrangement of the optical waveguides, leading to the generation of crosstalk.